Gentlemen, when you are in the position that was posed here initially, and that I, too, recounted, then you will have no doubts about the approach. Slant range doesn't even come into it! If you are in widespread fog then, yes, agreed, take extreme care as you might be 'suckered in' and fall foul of the slant range etc - but the situation described was completely different. We were in CAVOK conditions, as was the rest of the county but a little light fog was rising from the damp grass adjacent to the runway. This is quite common in the UK near rivers, streams etc. Fog was not forecast and neither was it occurring except for a few feet around the transmissometer. This post is all about what to do when clearly there is a paradox from the RVR transmissometers.....they happen to be poorly placed and are very obviously giving erroneous readings (well, they might be giving a correct reading for their immediate environment but if they reside in the only small packet of fog then you have a problem!)
Without a sensible ATCO then you will be damned by the authority for making the approach and you might be damned by the company for not 'taking the common sense decision'! The only answer, insofar as I can see is to elicit the ATCOs help and get it declared u/s. Believe me, you will not be thinking 'slant range' etc in the described conditions; more like "WTF! Why doesn't he look out of the window....." This phenomenon is much more widespread than you might think in the UK in the autumn on those beautifully crisp, clear and cold days.